The present invention relates to blends of one or more metal cation neutralized acid copolymers with one or more metal cation neutralized ethylene acrylates, and to improved golf ball covers made from these blends. More particularly, the invention relates to golf ball covers produced from blends of at least one metal cation neutralized olefin-carboxylic acid copolymer, such as ethylene acrylic or methacrylic acid copolymer, and at least one metal cation (preferably alkali metal cation) hydrolyzed and neutralized ethylene alkyl acrylate copolymer. The improved golf ball covers are useful for producing golf balls exhibiting the desired travel distance while maintaining or improving the playability and/or durability characteristics necessary for repetitive play.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E. I. DuPont de Nemours and Company under the trademark xe2x80x9cSurlyn(copyright)xe2x80x9d and more recently, by the Exxon Corporation under the trademarks xe2x80x9cEscor(copyright)xe2x80x9d and the tradename xe2x80x9cIotek(trademark)xe2x80x9d, have become the materials of choice for the construction of golf ball covers over the traditional xe2x80x9cbalataxe2x80x9d (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability properties required for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid or maleic acid. In some instances, an additional softening comonomer can also be included to form a terpolymer. The pendent ionic groups in the ionomeric resins interact to form ion-rich aggregates contained in a non-polar polymer matrix. The metal ions, such as sodium, zinc, magnesium, lithium, potassium, calcium, etc. are used to neutralize some portion of the acid groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc. for golf ball construction over balata.
Broadly, the ionic copolymers comprise one or more alpha-olefins and from about 9 to about 30 weight percent of alpha, beta-ethylenically unsaturated mono- or dicarboxylic acid, the basic copolymer neutralized with metal ions to the extent desired. Usually, at least 20% of the carboxylic acid groups of the copolymer are neutralized by the metal ions (such as sodium, potassium, manganese, zinc, lithium, calcium, nickel, magnesium, and the like) and exist in the ionic state. In general, ionic copolymers including up to 16% acid are considered xe2x80x9clow acidxe2x80x9d ionomers, while those including greater than 16% acid are considered xe2x80x9chigh acidxe2x80x9d ionomers by the present inventors.
Suitable olefins for use in preparing the ionomeric resins include ethylene, propylene, butene-1, hexene-1, and the like. Unsaturated carboxylic acids include acrylic, methacrylic, ethacrylic, xcex1-chloroacrylic, crotonic, maleic, fumaric, itaconic acids, and the like. The ionomeric resins utilized in the golf ball industry are generally copolymers of ethylene with acrylic (i.e. Escor(copyright) or Iotek(trademark)) and/or methacrylic (i.e. Surlyn(copyright)) acid. In addition, two or more types of ionomeric resins may be blended into the cover compositions in order to produce the desired properties of the resulting golf balls.
Along this line, the properties of the cover compositions and/or the ionomeric resins utilized in the golf ball industry vary according to the type and amount of the metal cation, the molecular weight, the composition of the base resin (i.e. the nature and the relative content of the olefin, the unsaturated carboxylic acid groups, etc.), the amount of acid, the degree of neutralization and whether additional ingredients such as reinforcement agents or additives are utilized. Consequently, the properties of the ionomer resins can be controlled and varied in order to produce golf balls having different playing characteristics, such as differences in hardness, playability (i.e. spin, feel, click, etc.), durability (i.e. impact and/or cut resistance), and resilience (i.e. coefficient of restitution).
However, while there are currently more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups), the degree of neutralization and additive ingredients such as reinforcement agents, etc., a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the playability characteristics previously associated with the balata cover, but also the improved impact resistance and carrying distance properties produced by the ionomeric resins. Thus, an object of the present invention is to provide golf ball cover compositions which, when utilized in golf ball construction, produce balls exhibiting the desired travel distance (i.e., similar coefficient of restitution values) while maintaining or improvity satisfactory playability and durability properties.
A golf ball""s coefficient of restitution (C.O.R.) is the ratio of the relative velocity of the ball after direct impact to that before impact. One way to measure the coefficient of restitution is to propel a ball at a given speed against a hard massive surface, and measure its incoming velocity and outgoing velocity. The coefficient of restitution is defined as the ratio of the outgoing velocity to incoming velocity of a rebounding ball and is expressed as a decimal. As a result, the coefficient of restitution can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision.
The coefficient of restitution of a one-piece golf ball is a function of the ball""s composition. In a two-piece or a multi-layered golf ball, the coefficient of restitution is a function of the core, the cover and any additional layer. While there are no United States Golf Association (U.S.G.A.) limitations on the coefficient of restitution values of a golf ball, the U.S.G.A. requires that the golf ball cannot exceed an initial velocity of 255 feet/second. As a result, golf ball manufacturers generally seek to maximize the coefficient of restitution of a ball without violating the velocity limitation.
In various attempts to produce a high coefficient of restitution golf ball exhibiting the enhanced travel distance desired, the golfing industry has blended various ionomeric resins. However, many of these blends do not exhibit the durability and playability characteristics necessary for repetitive play and/or the enhanced travel distance desired.
It is, therefore, desirable to develop cover compositions which produce golf balls exhibiting properties of desired carrying distance (i.e., possess desirable coefficient of restitution values) such as the ionomeric cover blends set forth in U.S. Patent Nos. 4,884,814 and 4,911,451, without sacrificing or improving playability and/or durability characteristics.
Furthermore, while as stated above, Surlyn(copyright) and Escor(copyright) (or xe2x80x9cIotek(trademark)xe2x80x9d) are materials of choice for golf ball cover construction when balata (natural or synthetic) is not used, these materials are relatively costly and available from limited sources. It has, therefore, become desirable to develop alternative ionomer resin based compositions having properties suitable for use to in golf ball cover construction.
These and other objects and features of the invention will be apparent from the following description and from the claims.
The present invention is directed to golf ball covers, and more specifically to golf ball cover compositions which comprise blends of one or more metal cation neutralized olefin-carboxylic acid copolymers, such as ethylene acrylic or methacrylic acid copolymers, and one or more metal cation (preferably alkali metal cation) neutralized ethylene alkyl acrylate copolymers. The golf ball covers of the invention provide desirable alternatives to covers produced from the Surlyn(copyright) and Iotek(trademark) ionomeric resins currently used in golf ball cover construction.
The present invention is particularly directed to golf ball cover compositions which are prepared by blending at least one metal cation neutralized acid copolymer which contains about 1% to about 30% carboxylic acid, with at least one alkali metal cation neutralized and hydrolyzed and neutralized ethylene alkyl acrylate copolymers containing up to about 30% by weight of an alkyl acrylate. The alkyl acrylate in the ethylene copolymer is any acrylic or methacrylic esters of linear, branched or cyclic alkanols having 1-28 carbon atoms including, among others, ethyl acrylate, methyl acrylate and butyl acrylate. The ethylene copolymers may, therefore, comprise ethylene ethyl acrylate (EEA), ethylene methyl acrylate (EMA), and/or ethylene butyl acrylate (EBA) among others.
The selection of type and ratio of the metal cation neutralized acid copolymer, such as ethylene acrylic acid copolymer, ethylene methacrylic acid copolymer, etc., to the alkali metal cation neutralized ethylene alkyl acrylate copolymer (i.e., EEA, EMA, EBA, etc.) is based on the desired final hardness, durability, travel distance and spin rate of the ball. Preferably, the blend of the metal cation neutralized acid copolymer and the alkali metal cation neutralized ethylene alkyl acrylate includes about 0 to 90 parts by weight acid copolymer, and about 100 to 10 parts by weight ethylene alkyl acrylate. More preferably, the blend is comprised of about 0 to 75 parts by weight partially neutralized acid copolymer and about 100 to 25 parts by weight hydrolyzed and neutralized ethylene alkyl acrylate. Most preferably the blend consists of about 0 to about 50 parts by weight neutralized acid copolymer and about 100 to 50 parts by weight neutralized and hydrolyzed ethylene alkyl acrylate.
In another aspect, the invention relates to a cover composition comprising a blend of i) a metal cation neutralized copolymer of about 1 weight percent to about 30 weight percent, preferably 9 to 25 weight percent of an alpha, beta-unsaturated carboxylic acid (preferably acrylic acid or methacrylic acid) and an olefin (preferably ethylene), and ii) an metal cation (preferably alkali metal cation) neutralized ethylene alkyl acrylate (preferably ethylene methyl acrylate and ethylene butyl acrylate). Approximately, 10% to about 90% of the carboxyl groups of the acid copolymer are neutralized with a metal cation such as a metal cation selected from the group consisting of manganese, lithium, potassium, calcium, manganese, zinc, sodium, and nickel. In addition, the carboxylic acid esters of the ethylene alkyl acrylate copolymers are hydrolysized or neutralized though the use of alkali metal cations.
In another further aspect, the invention relates to a golf ball comprising a core and a cover, wherein the cover is comprised of a blend of i) a metal cation neutralized copolymer of about 1 to 30% by weight of an acrylic acid and the remainder ethylene and ii) an alkali metal cation neutralized ethylene alkyl acrylate. Approximately, 10% to 90% of the carboxyl groups of the acrylic acid/ethylene copolymer are neutralized with a metal cation such as a metal cation selected from the group consisting of manganese, lithium, potassium, sodium, zinc, magnesium, calcium and nickel. Moreover, the carboxylic acid esters of the ethylene alkyl acrylate copolymer are hydrolysized and neutralized by an alkali metal cations such as lithium, sodium and potassium. In addition, the cover may contain one or more additional ingredients such as pigments, dyes, U.V. absorbers and optical brighteners. The core is generally a solid core.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.